The present invention relates to a method for reducing the excess sludge generated in the biological treatment of various types of organic waste fluid, and an apparatus for reducing excess sludge using this method.
The organic waste fluid which is discarded in the form of various types of industrial waste fluids as well as domestic sewage is primarily treated by biological methods such as by using activated sludge. Since the microbes used can proliferate in abundance during this process, creating massive amounts of excess sludge whose disposal can present a problem. As methods for handling this type of excess sludge, it has been used for improving soil or as compost, but no fundamental solution has yet been reached, so that in most cases, the excess sludge is dehydrated and incinerated or buried as industrial waste.
However, in recent years, such incineration has come to be seen as possibly posing a threat by generating toxic substances including environmental hormones such as dioxins, and burial ultimately also gives rise to similar problems in the form of toxic substances which can leak out over time. Therefore, there has been a call for development of techniques for reducing the excess sludge itself to provide a more fundamental solution to the problem, and many proposals have been offered.
Among these, the methods of Japanese Patent No. 2806495, Japanese Patent Application, First Publication No. H11-128975 and Japanese Patent Application No. H11-218022 are relatively inexpensive and easy to control. All of these methods involve the addition of alkalis to the excess sludge followed by exposure to ultrasonic waves.
However, the method of Japanese Patent Application, First Publication No. H11-128975 is performed at a pH of 10.5 or less in consideration of the cost of neutralization that is required when returning the solubilized excess sludge to the biological treatment, so that the expected solubilization effect is difficult to achieve with this method.
The method of Japanese Patent Application No. H11-218022 enables the pH to be raised to 12-13 by improving on the neutralization procedure, thus improving the solubilization effect, but a lot of time is still required for the ultrasonic treatment, which requires high output and is therefore very expensive. Additionally, the ability of microbes to be used for decomposition leaves something to be desired, to the point of being difficult to put into actual practice. In connection with the art described above, there has been a strong demand for technological improvements that would allow for cost reductions and improved effectiveness by obtaining the desired level of solubilization effects, and particularly the ability of microbes to be used for decomposition, even with a low ultrasonic output and a short time of application thereof.
The present invention has the object of offering a method and apparatus for solubilizing excess sludge, with a higher capacity for solubilization than conventional methods, having a low overall cost and allowing equipment to be made smaller.
A method for reducing excess sludge according to claim 1 of the present invention is characterized by adding a solubilizing agent to excess sludge generated by a microbial treatment of organic sewage; applying ultrasonic waves; applying a reduced pressure swelling treatment; and returning the result to the microbial treatment, thereby reducing the volume of said excess sludge.
Due to this method, a high level of solubilization can be obtained by the reduced pressure swelling process, thus resulting in a particularly high level of reusability by microbes, and achieving a high rate of reduction.
Another method for reducing excess sludge according to the present invention is characterized in that said solubilizing agent is an alkali, a bacteriolytic agent, or a combination thereof.
According to this method, the alkalis contribute to the dissolution of proteins, are not harmful to the environment if neutralized, and further in biological treatment systems, and can be used to adjust the pH prevent reductions in pH due to dissolution of carbon dioxide gas. While possible alkalis include, for example, NaOH, KOH, Mg(OH)2 and Ca(OH)2, other compounds may be used as long as they are capable of raising the pH.
Additionally, the bacteriolytic agent has the function of destroying the cell walls of microbes. While there are many types of bacteriolytic agents, the type does not matter as long as it is capable of decomposing bacteria. Examples include hydrogen peroxide, dichlorous soda and ozone.
An apparatus for reducing excess sludge according to another embodiment of the present invention is characterized by comprising means for adding a solubilizing agent to excess sludge generated by microbial treatment of organic sewage; ultrasonic wave applying means for applying ultrasonic waves; and reduced pressure swelling means, provided downstream of the ultrasonic wave applying means, for applying a reduced pressure swelling process to the excess sludge.
According to this embodiment, a high rate of solubilization is achieved by the reduced pressure swelling means, thus resulting in a particularly high level of reusability by microbes, and achieving a high rate of reduction.
An apparatus for reducing excess sludge according to another embodiment of the present invention is characterized in that the reduced pressure swelling means is a homogenizer.
According to this embodiment, solubilizing agents such as alkalis and ultrasound are used to destroy or damage the cell walls of microbes, and a reduced pressure swelling process causes the contents of the cells to leak out, after which further reactions by solubilizing agents causes them to be converted to substances which can be handled by microbes. Whereas the solubilization effect due to the reduced pressure swelling can be raised by raising the pressure of the homogenizer, the optimum operating conditions may be determined in consideration of the manufacturing cost and running cost of the apparatus.
An apparatus for reducing excess sludge according to another embodiment of the present invention is characterized in that said ultrasonic wave applying means and said reduced pressure swelling means are constructed as separate parts, the ultrasonic wave applying means and reduced pressure swelling means being serially connected directly or through the medium of other equipment.
According to this embodiment, the ultrasonic wave applying means and the reduced pressure swelling means are separate parts, thus allowing for a design which maximizes the performance of the respective parts. Furthermore, the operating conditions of the processes of application of ultrasonic waves and reduced pressure swelling can be freely controlled independently for respective optimization.
An apparatus for reducing excess sludge according to yet another embodiment of the present invention is constructed as a single apparatus having the functions of both said ultrasonic wave applying means and said reduced pressure swelling means.
According to this embodiment, the ultrasonic wave applying means and reduced pressure swelling means are constructed as a single apparatus, thus allowing for a design capable of holding the size of the apparatus to a minimum. Additionally, the cost of the apparatus overall can be reduced, thus enabling the apparatus to be easily controlled as well.
An apparatus for reducing excess sludge according to yet another embodiment of the present invention is characterized in that the ultrasonic wave applying means includes an ultrasonic vibrator, and said reduced pressure swelling means comprises a plate having a plurality of through holes formed therein, provided at a position downstream of said ultrasonic vibrator such as to intersect flow.
According to this embodiment, the plate (porous plate) positioned against the flow gives rise to the reduced pressure swelling effect, thereby ensuring a high level of solubilization.
An apparatus for reducing excess sludge according to yet another embodiment of the present invention is characterized in that a flow-receiving plate is provided between said ultrasonic wave applying means and said reduced pressure swelling means so as to obstruct flow.
According to this embodiment, the flow-receiving plate that is provided on the flow path increases the ultrasonic wave cavitation effect, changes the flow of the solution, and aids in mixture, so that a high rate of solubilization can be obtained even if the plate (porous plate) is given a larger hole diameter.